PROJECT SUMMARY Sexual reproduction requires the creation of haploid gametes from diploid cells. This two-fold reduction in chromosome number is carried out by a specialized, evolutionarily conserved, cell division called meiosis. In humans, failures in meiosis result in infertility and birth defects such as Trisomy 21 or Down syndrome. Unique to meiosis is the first meiotic division, in which homologous pairs of sister chromatids segregate to opposite poles. To promote proper orientation at Metaphase I of meiosis, homologous chromosomes are physically connected by a combination of sister chromatid cohesion and reciprocal crossovers. Crossovers result from meiotic recombination, which is initiated by double strand breaks (DSBs). Repair of these DSBs brings homologs together to form a structure called the synaptonemal complex (SC). Meiotic DSB repair is highly regulated to ensure that each homolog pair gets at least one crossover and that chromosome segregation is delayed until all DSBs have been repaired. An important element of this regulation in budding yeast involves phosphorylation by the meiosis-specific kinase, Mek1, as well as the conserved cell cycle kinases, CDK (cyclin-dependent kinase), DDK (Cdc7-Dbf4) and polo-like kinase (Cdc5). The goal of this grant is to understand how phosphorylation regulates meiotic recombination and chromosome synapsis. The first aim tests a specific mechanistic hypothesis for how meiotic DSB repair is coordinated with meiotic progression through Mek1 phosphorylation of the meiosis-specific transcription factor, Ndt80. The second aim addresses the role that phosphorylation of a conserved region of an SC protein called Zip1 plays in regulating in the crossover/noncrossover decision by enabling the creation of a specific class of crossovers that are distributed throughout the genome. The third aim takes advantage of a major resource we developed in the last grant period?namely a dataset containing thousands of phosphorylated amino acids on proteins arrested in meiotic prophase. Phosphosites on proteins important for meiotic recombination and synapsis will be mutated and phenotypically characterized to discover the functional role of the phosphorylation. Two examples are provided for proteins we plan to pursue in the near future: Ecm11?a SUMOylated protein that is necessary for SC formation and Red1, whose degradation is mediated by Cdc5, resulting in SC disassembly and inactivation of Mek1 to allow repair of residual DSBs prior to the onset of the first meiotic division.